George T. Wolff

On the nature of nitrate in coarse continental aerosols

In nonmarine environments, the nitrate (NO3−) aerosol occurs in both the fine fraction (diameter < 2.5 μm) as NH4NO3 and in the coarse fraction (diameter ⩾ 2.5 μm). Our summertime studies in the eastern U.S. show that NO3− is predominantly in the coarse fraction. Strong correlations between coarse NO3− and the crustal elements suggest that the NO3− is associated with entrained soil dust. Various formation mechanisms, including artifact NO3− formation on filters, were examined and it appears that the most important mechanism is the reaction of HNO3 with airborne crustal constituents.

The Denver Winter Aerosol: A Comprehensive Chemical Characterization

The sampling and chemical analysis of the ambient aerosol collected in Denver, CO, for a 40-day period during November and December, 1978 are described in this report. Parameters included 12-hr TSP measurements, 24-hr respirable and inhalable mass measurements, and 4-hr measurements of mass and chemical species (NO3−, SO4 =, NH4 +, organic and elemental carbon as well as 13 chemical elements) in two size fractions i.e., less than 2.5 μm diameter (fine fraction) and larger than 2.5 μm diameter (coarse fraction). On the basis of the chemical analyses, it was possible to account for all particulate mass in both size fractions. In the fine fraction, the major constituents were organic carbon (21.6%), NH4NO3 (20.0%), elemental carbon (15.3%), (NH4)2SO4 (13.6%), and the remainder consisted primarily of soil-like material, lead salts, and adsorbed water. Three quarters of the coarse fraction consisted of soil-like material, with the remainder composed of the same species that dominated the fine fraction.

Measurements of SOx, NOx and aerosol species on Bermuda

During August 1982 and January and February 1983, General Motors Research Laboratories operated an air monitoring site on the southwest coast of Bermuda. The data show that the levels of the NOx and SOx species reaching Bermuda are determined by the direction of the air flow. The highest levels of sulfate (mean = 4.0 μg m−3), nitric acid (126 ppt) and other species are observed when air masses arrive from the northeastern United States while the lowest levels (sulfate = 1.1 μg m−3; nitric acid = 41 ppt) occur during air flow from the SE direction. With westerly air flow, increases in many anthropogenic constituents such as particulate sulfate, lead, elemental carbon, sulfur dioxide, nitrogen dioxide, nitric acid and ozone are observed. These species are generally the lowest during SE winds which bring high concentrations of soil- and crustal-related aerosol species. The source of this crustal material appears to be the Sahara Desert. On the average, the levels of anthropogenic constituents are higher in winter because of frequent intrusions of N American air masses. Conversely, the levels of crustal materials are higher in summer when the SE flow is more prevalent.

Sources and sinks of ozone in rural areas

Abstract Based on data collected at rural sites in South Dakota, Louisiana and Virginia during the summers of 1978–1980, the factors controlling the diurnal behavior of O3 near the surface were determined. At all three sites, the diurnal O3 profile consisted of a period of O3 increase from sunrise until mid-afternoon, and a period of O3 decrease from late afternoon until sunrise. The daily O3 increase was comprised of two approximately equal portions: one due to downmixing of O3 from aloft (0600–1000 h), and the other due to photochemistry and possibly some further downmixing (1000–1400 h). Even assuming that all of the O3 increase during the 1000–1400 h period was due to local photochemistry, only very small amounts of O3 were formed i.e. about 6 ppb of O3 per ppb of nitrogen oxides (NOx). Both bag irradiation and photochemical modeling studies suggested similar, but slightly higher O3 formation. Comparing the ambient data with the bag irradiation and modeling results, it was concluded that rural photochemistry was not controlled exclusively by NOx, but depended on hydrocarbons as well. The O3 decrease (1800 to 0600 h) generally occurred while nocturnal inversions isolated the lower 50 m of the atmosphere from the air aloft. Since the O3 loss-rate could be parameterized equally well by either zero- or first-order rate laws, we could not distinguish between the two mechanisms. However, concentrations of gaseous species such as terpenes and NOx that might lead to pseudo zero-order O3 loss were generally of minor significance. Consequently, it is concluded that O3 loss was controlled by surface deposition with deposition velocities ranging from 0.06 to 0.34cms−1 at the three sites.

Prediction of ambient light scattering using a physical model responsive to relative humidity: Validation with measurements from detroit

Abstract A model for the prediction of atmospheric light extinction from field measurements of airborne aerosol concentration, chemical speciation, and size distribution has been tested with field data from the Detroit, Michigan area. The amount of light scattered by airborne particulates in ambient conditions is accurately predicted by the model using measurements of accumulation mode sulfate, nitrate, volatile and nonvolatile carbon. Model predictions were compared with simultaneously measured light scattering. The model is unique amongst physical models in its simulation of the effect of relative humidity on visibility reduction and on the light scattering efficiency of specific chemical constituents of the airborne aerosol. A proven physical model offers an advantage over statistical models in that it should prove reliable for extrapolation of ambient conditions well outside conditions currently realized. Within the range of the available data, the physical model is shown to simulate the light scattering as well as statistical models. Calculations with the physical model indicate that on average sulfate compounds are less efficient scatterers of light in ambient settings than statistical analyses would indicate.

Weekday/Weekend Ozone Differences: What Can We Learn from Them?

Abstract A national analysis of weekday/weekend ozone (O3) differences demonstrates significant variation across the country. Weekend 1-hr or 8-hr maximum O3 varies from 15% lower than weekday levels to 30% higher. The weekend O3 increases are primarily found in and around large coastal cities in California and large cities in the Midwest and Northeast Corridor. Both the average and the 95th percentile of the daily 1-hr and 8-hr maxima exhibit the same general pattern. Many sites that have elevated O3 also have higher O3 on weekends even though traffic and O3 precursor levels are substantially reduced on weekends. Detailed studies of this phenomenon indicate that the primary cause of the higher O3 on weekends is the reduction in oxides of nitrogen (NOx) emissions on weekends in a volatile organic compound (VOC)-limited chemical regime. In contrast, the lower O3 on weekends in other locations is probably a result of NOx reductions in a NOx-limited regime. The NOx reduction explanation is supported by a wide range of ambient analyses and several photochemical modeling studies. Changes in the timing and location of emissions and meteorological factors play smaller roles in weekend O3 behavior. Weekday/weekend temperature differences do not explain the weekend effect but may modify it.

The Nature and Sources of Haze in the Shenandoah Valley/Blue Ridge Mountains Area

In order to investigate the nature and sources of regional haze, the General Motors mobile Atmospheric Research Laboratory was used in the summer of 1980 to monitor ambient air quality in the Shenandoah Valley of northern Virginia. On the average, 92% of the total light extinction was due to scattering by particles; the remainder was due to scattering by gases and absorption by gases and particles. Sulfate aerosols were the most Important visibility-reducing species. Averaging 55% of the fine participate mass, sulfates (and associated water) accounted for 78% of the total light extinction. The second most abundant fine particulate, accounting for 29% of the fine mass, was carbon—most of which was organic. Most of the remaining particulate mass and extinction were due to crustal materials. It is estimated that 78–86% of the total light extinction was caused by anthropogenic aerosol, most of which originated in major source areas of the midwest.

The flux of S and N eastward from North America

Using climatological wind data and average concentrations of SO2, SO4, NOx NO3, HNO3 and PAN from data bases in the United States and Canada we calculate the net atmospheric transport of S and N eastward from North America in eight latitude and three altitude classes. A broad maximum in transport occurs between 33° and 48°N. Of the S and N emitted to the atmosphere of eastern North America, about 34 and 24–71%, respectively, are advected eastward. Transport at all levels between 0 and 5500 m is important.

Particulate Elemental Carbon in the Atmosphere

This report summarizes the important results which were presented at an international symposium on particulate elemental carbon in the atmosphere. The “Particulate Carbon: Atmospheric Life Cycle” symposium was sponsored by General Motors Research Laboratories on October 12–14, 1980. Supplemental background data are included.

Chemical analysis of size-segregated samples of Denver's ambient particulate

Size-segregated impactor samples of Denvers wintertime airborne particulate were analyzed for the major chemical species. These species are responsible for most of the pollution-related visibility reduction in Denver. Samples were collected three times a day during a 40-day field program in November-December, 1978. Each of the measured chemical species (sulfate, nitrate, ammonium, bromide, organic and elemental carbon) had a log-normally distributed accumulation mode with a mass median aerodynamic diameter near 0.3 ..mu..m. For all species, particle size increased with increasing relative humidity. These results are supported by electrical aerosol analyzer (EAA) measurements. The impactor results for this time period are consistent with a photochemical mechanism for particulate nitrate formation and a heterogeneous mechanism for the formation of sulfate particles. 23 references, 5 figures, 5 tables.